I don't think the games would have been successful had there been no new Pokemon. There are no Pokemon that could replace Reshiram and Zekrom (and much less Kyurem), so the plot would have been completely different. The pre-Gen 5 Pokemon have already seen their share of the limelight, and there really isn't much else that can be done with their storylines; if anything, the Pokemon would have to have more detail added to them, thus changing what is already established about them, which in turn would probably be even worse-received than the BW games we got.

Honestly, the only complaints I've seen about BW are: that it had little to no post-game, and that people didn't like the new Pokemon and couldn't use old ones. The post game has nothing to do with what Pokemon are available; and not liking the new Pokemon and being dependent only on old ones will certainly turn into a gen1er argument that I'd rather not get into. So had BW not introduced new Pokemon, I'm certain that it would not have been successful; one of BW's best traits is its plot, and that plot could not exist completely if new Pokemon were not introduced.